The present invention relates to a method of driving a drop-on demand type ink-jet head of a multi-drop system which merges a plurality of ink-drops sequentially emitted from an orifice of an ink chamber to form a single liquid drop.
There has been a method of expressing gradations by using an ink-jet head which performs printing in such a manner in which a plurality of ink chambers are provided, drive pulse voltages are applied to piezoelectric members provided in correspondence with the ink chambers so as to cause the piezoelectric members to be transformed, and the ink chambers are selectively deformed by the transitions of the piezoelectric members so as to emit ink from the ink chambers. A method known as the kind of method described above is, for example, a method of expressing gradations in which sizes of ink drops hit on a recording medium are changed by controlling volumes of ink drops to be emitted by P.W.M. (Pulse-Width-Modulation) control, as disclosed in U.S. Pat. No. 5,461,403, or a method of expressing gradations in which a plurality of ink drops are emitted sequentially from one same orifice and the number of ink drops hit on one same portion on a recording medium is controlled.
The former method has a problem that the emission volumes of ink drops are not constant unless the following ink drop is emitted under a condition that meniscus of the orifice is recovered and stabilized to some extent after emission of a previous ink drop from the orifice. Therefore, the driving frequency must be lowered, so that the printing speed is difficult to be increased. In contrast, the latter method of a multi-drop system is advantageous in that it is possible to improve the printing speed by increasing the driving frequency and that small liquid drops can be emitted without reducing the emission speed. However, since a line head performs printing while moving a recording medium in the sub-scanning direction, for example, there is a problem that the recording medium itself is moved and the printed dots are elongated in the moving direction of the recording medium while sequentially emitting seven liquid drops in case of performing one-dot printing by emission of seven liquid drops within a drop-on-demand drop production period.
A method of solving the above problems is, for example, that the speed of ink drops to be emitted later is gradually increased to be higher than the speed of the ink drop to be emitted first, such that the ink drops emitted later catch up with and are merged with the ink drops emitted earlier to obtain one liquid drop therefrom when the ink drops hit on a recording medium, as disclosed in U.S. Pat. No. 5,285,215. This is realized by applying successive drive pulse voltages to a piezoelectric member such that the amplitude of a pressure wave in an ink chamber is gradually increased when emitting ink.
However, in this case, the vibration amplitude of an ink chamber increases and the vibration of the liquid surface (or meniscus) at an orifice is increased accordingly since the emission speed of ink drops to be emitted later is increased. Meanwhile, printing cannot be started until the vibration of the meniscus of the orifice ceases, where printing for a next line is started after printing for one line is completed. If the vibration of the meniscus of the orifice is large, the time required until the vibration ceases is elongated. As a result, there is a problem that the printing speed cannot sufficiently be increased.